Recently, cloud data centers have been established by providers of cloud services such as Infrastructure as a Service (IaaS). In each of the cloud data centers, a large number of physical machines (servers) are arranged and a plurality of virtual machines are provided on each of the physical machines by a virtualization technique.
Operations of a customer who uses a cloud service are achieved by a system built by one or more virtual machines. The customer uses a terminal connected to the virtual machines through a network such as an intranet and performs the operations.
A service provider that has established a cloud data center makes a service level agreement (SLA) with each of customers. Service level evaluation items for evaluating performance such as a response time, the maximum number of simultaneous possible connections, and utilization, requested values of the evaluation items for evaluating performance, penalty amounts, and the like are defined in the SLA in order to satisfy requests for the contents and quality of a service to be provided. The service provider operates and monitors virtual machines on the basis of the SLA.
The number of virtual machines to be provided on a single physical machine is determined so that the total of resources of the provided virtual machines does not exceed an upper limit on an available resource provided by the physical machine. This is due to the fact that if all the virtual machines are executed and the total of resources used by the virtual machines exceeds the upper limit on the available resource provided by the physical machine, a requested value of an evaluation item defined in the SLA may not be satisfied. It is, however, rare that the virtual machines are executed for a long time while using the resource of the physical machine so that the total of the resources used by the virtual machines exceeds the upper limit on the available resource provided by the physical machine. Recently, overcommit has been used, in which virtual machines are provided so that the total of resources of the virtual machines exceeds an upper limit on an available resource provided by a physical machine.
For example, when the capacity of a memory of a certain physical machine is 64 GB, virtual machines that use a memory with the maximum capacity of 98 GB may be provided on the certain physical machine.
Japanese Laid-open Patent Publications Nos. 2003-223335, 2008-140240, and 2011-39740 are examples of related art.